The present invention relates to control mechanisms for rotating airframes, and more particularly, to a steering rate sensing device and active damping circuit for use in an autopilot control system which directs the flight path maneuvers of a rolling missile.
Many missiles have been designed for intentionally induced and maintained roll rates about their longitudinal axis during flight. Such missiles have significant practical advantages over roll stabilized airframes. This rolling airframe concept has been applied to both air and surface launched missiles. These missiles can be spun initially by the launcher and utilize control surfaces to maintain a predetermined rate of roll. With a roll rate of approximately 5 to 10 revolutions per second, it is possible to utilize a single control plane to guide the missile in all three earth related axes.
In a typical application of this concept, as disclosed in U. S. Pat. No. 4,037,806, the control system utilizes a single pair of variable incidence control surfaces to steer the missile about the control plane at a selected instantaneous rotational orientation upon command from a guidance command signal. Thus, with such a missile operating in a level flight attitude, to cause the missile to climb, a guidance command signal must vary in amplitude at a frequency equal to the roll rate of the missile. For example, in the vertical plane, the guidance command signal would be a generally sinusoidal wave form that would induce pitch-up as the control plane of the vehicle approaches earth vertical and pitch-down after the control surface rotates and nearest a one-half revolution from pitch-up, thereby producing upward change in the angle of attack. The angle of attack produces a body lift and alters the missile course from a horizontal to a climbing course. Similarly, a course change to the right would be effected by a sinusoidal signal displaced 90.degree. from the signal required for a vertical course change. This provides a simplified control system resulting in a reduction in cost and an increase in reliability for rolling airframes in contrast with stabilized airframes.
The present invention was conceived and developed for utilization in a recently developed autopilot control system for rolling airframes which is disclosed in U.S. Pat. No. 4,054,254. In such a control system, it is desirable to produce a damping of the commanded wing incidence to prevent overshoot.
In U. S. Pat. No. 4,054,254 mentioned above, the steering rate sensing device includes a pivotally mounted magnetic flapper surrounded by an inductive pick-off assembly. The flapper is immersed within a damping fluid. Since the sensing device rotates with the airframe, a gyroscopic effect is produced on the flapper which in conjunction with the damping fluid stabilizes the position of the magnetic flapper, and therefore a zero output is produced by the inductive pick-off assembly. However, when action of the control surfaces causes the airframe to pitch in the control plane, the angular velocity of that pitching movement determines the degree to which the flapper will precess. This causes the magnetized flapper to approach the inductive pick-off assembly and produce a signal output corresponding to the angular velocity on pitch rate. The output of the pitch rate sensing device is summed with the undamped control signal to produce a damped control signal. This prevents overshoot.
Steering rate sensing devices which may be utilized in the autopilot control system of the aforementioned U.S. Pat. No. 4,054,254 are disclosed in U.S. Pat. Nos. 4,114,451 and 4,114,452. These devices may also be fluid damped.
In prior steering rate sensing devices, the degree of damping of the rotor, i.e., the damping coefficient, must be carefully controlled to achieve missile flight path accuracy. This is because the output of such steering rate sensing devices is proportional to the damping. Prior steering rate sensing devices have been subject to large variations in output with changes in temperature. This is due to fluid viscosity changes in the case of fluid damped devices and due to changes in resistivity in the case of electromagnetically damped devices.